In graphic recording of electrical signals, for example facsimile signals electrically marked during line by line scanning across a moving web, it is not only necessary that the applied facsimile signals received from independent equipment occur at the same rate as the line by line scanning of the web, it is also necessary that sucessive lines of received signals be in phase with the line scan of the web. That is, the start of each line of facsimile signals must be brought into coincidence with the start of line recording scan of the web so that the recorded information is framed on the web. The process of bringing the scanning of the web into phase with the incoming signals is termed framing.
In facsimile recorders scanning of electrosensitive paper or like web is effected by scan means moved or commutated by a driver. For example, in electrolytic recorders a motor drives a helical electrode curved around a drum or a number of styli carried on an endless belt. It is customary to derive a phasing pulse from the scan means, which pulse is called a recorder pulse or start of line pulse or signal to distinguish it from the framing pulse or signal which constitutes a part of the incoming facsimile signal. One form of facsimile framing signal is a series of substantially continuous sinusoidal modulated lines interrupted or altered for a short interval between each line. This framing signal or tone is transmitted for seconds before the transmission of graphic facsimile information, and a framing pulse can be derived from the interval between each line of tone for comparison with the recorder pulse or framing signal.
Typically the recorder pulse or framing signal has been derived from the recorder scan means by electromechanical or electromagnetic means. For example, a scanning helix drum or a stylus on a scanning belt can generate a pulse just before each scan of a recording web by momentarily closing a switch mechanically or magnetically. Mechanical switches closed millions of times are themselves notoriously subject to failure, and may undesirably affect the recording characteristics of the stylus. Magnetic reed switches are relatively free from wear but are difficult to adjust for precise closure at a selected position of the scan means. And since the preferred metals for stylii, e.g. stainless steel, are weakly-or non-magnetic, they cannot be used with reed switches.
Accordingly, the objects of the present invention are to provide a way of generating a graphic recorder framing signal, which avoids use of electromechanical or electromagnetic devices and is free of their adjustment and maintenance problems, and which utilizes the available interrupted framing tone of facsimile signals.